The present invention relates to automatic equalizers which compensate for the distorting effects of bandlimited channels on transmitted data signals.
Automatic equalizers are necessary for accurate reception of high speed data signals transmitted over bandlimited channels with unknown transmission characteristics. The equalizer, which is resident in the receiver portion of a data set, or "modem", is generally in the form of a transversal filter. Samples of the incoming data signal, referred to herein as "line samples", are formed at a predetermined sampling rate. The line samples, which represent the value of the received signal at the time points at which they are taken, are applied to the filter, where they are multiplied by respective tap coefficients. The resulting products are added together and, if necessary, demodulated to generate what is hereinafter referred to as a "baseband equalizer output," or just "equalizer output." The equalizer output is thereafter quantized to recover the transmitted data. In addition, an error signal is formed equal to the difference between the equalizer output and a reference signal which represents the transmitted data symbol. In the so-called adaptive type of automatic equalizer, in particular, the reference signal is derived from the decision made in the receiver (on the basis of the equalized signal value) as to what data symbol was transmitted. The error signal is used to update the tap coefficient values in such a way as to minimize a measure of the distortion--assumed to be primarily intersymbol interference--introduced by the channel.
Before accurate data recovery can be assured, appropriate values for a number of equalizer operating parameters need to be established. These may include, for example, an initial set of tap coefficient values, the timing epoch and the demodulating carrier phase. These parameters are functions of, for example, the transmitter clock phase and the transfer function of the channel, neither of which is known at the receiver. Accordingly, a start-up signal sequence from which the appropriate parameter values can be determined at the receiver is transmitted thereto over the channel in question. Line samples of various signals within the start-up sequence are loaded into the equalizer and processed in the manner described above. The required parameter values are then arrived at in response to the resulting error signals and/or equalizer outputs.
It is desirable in most applications, and crucial in some, to minimize the length of the start-up period; the time consumed in the start-up process is time wasted from the standpoint of the transmission of message data. One of the factors contributing to the length of the start-up period is the time required to "load the equalizer," by which is meant the generation of a sufficient number of line samples that the above-described decision- and error-forming process--and thus determination of the equalizer operating parameters--can begin.